Brushless permanent DC motors are becoming increasingly popular, including for traction drive systems, that is for vehicles, such as golf carts, electric scooters, electric motorcycles, electric cars, slab crabs, electric outboard motors for boats, etc. In such an environment, high power density and high efficiency are the primary performance needs.
In order to achieve high power density and high efficiency for such applications, it is critical that an effective winding configuration of the stator coils, and an efficient cooling mechanism, are provided. Since traction drive applications are high power applications, the number of turns per stator slot is low and the percentage of resistance of the interconnections between the coils compared with total winding resistance is high if conventional lap winding configurations are used. Thus, conventional lap winding configurations increase copper losses, and reduce motor efficiency.
Conventional wave windings do not have the same interconnection resistance problems that lap windings do, but have large end turn problems. Because the end turns in conventional wave windings are not symmetrically distributed on both sides of the stator core there is an increase in the volume and weight of the motor, and reduced power density. The wave winding used in U.S. Pat. Nos. 5,592,731, 5,832,859 and 5,319,844 does not have the same problems as the conventional wave windings do, but is much more difficult to wind because one has to insert windings into slots one turn at a time.
The vast majority of conventional electric machines, including brushless permanent magnet DC motors, are housed inside of a motor housing. Typically, there is a set of cooling fins attached to the outside diameter of the housing. This conventional construction has several drawbacks, however. One drawback is that there is a contact thermal resistance between the stator and the motor housing. Another is a higher cost than desirable; because the machining of the distinct housing and stator housing assemblies is difficult because there are multiple components.
According to the present invention a brushless magnetic DC motor, and a stator assembly (and method of production thereof) for such motors, are provided which have a number of advantages compared to the prior art described above. The novel winding configuration according to the invention has no distinct interconnection wires between coils, and has much smaller end turns than conventional wave windings, resulting in higher power density and higher efficiency. The motor according to the invention also has a motor housing, cooling fins, and shroud (if provided) which are part of the stator, resulting in a reduced cost of construction, and enhanced cooling since there is no thermal gap, or thermal contact, resistance between a distinct housing and stator.
According to one aspect of the present invention, a brushless permanent magnet DC motor is provided comprising the following components: A stator having first and second sides. A rotor containing permanent magnets and radially spaced with respect to the stator to cooperate with the stator. A housing containing the stator and rotor. A stator winding having a plurality of coils, and end turns. And the end turns of the winding distributed on both the first and second sides of the stator, and the coils of the winding being unconnected to each other by distinct interconnection wires (that is the interconnections between the coils are part of the coils).
Preferably the windings comprises copper wires having a size between 26-32 AWG (i.e. about 0.0161-0.0079 inches in diameter), and the windings are impregnated with varnish to isolate the copper wires. However, no varnish is provided on machined surfaces. The stator has slots, and preferably the motor is a multiple (e.g. three) phase motor having multiple (e.g. four) coils, and the windings comprises one or more (e.g. four) turns per slot for each coil. The windings typically include lead wires, and the motor further comprises lacing cord lacing the end turns and the lead wires.
Preferably the stator includes the housing and integral, radially extending, circumferentially spaced cooling fins, so that there is no thermal gap, or thermal contact, resistance between distinct housing and stator components. Typically the rotor comprises a central shaft and a fan may be connected to the shaft for forcing air between the cooling fins to effectively receive heat transferred from the cooling fins, and carry the transferred heat away from the motor.
The motor also includes end caps having bearings mounting the shaft for relatively friction-free rotation with respect to the stator. Also, in order to provide optimized service life, means are provided for positively maintaining the bearings concentric with the stator. While any conventional structure may be utilized for positively maintaining the bearings concentric with the stator, preferably such means comprise the outside of the stator locating the radial position of the end caps; or when positioning rings are connected to the end caps then the positively positioning means may comprise fasteners extending between some of the spaced cooling fins and the positioning rings to connect the end caps together so that the fasteners locate the radial position of the end caps.
The motor according to the invention is typically adapted to be mounted in a vehicle for powering the vehicle.
According to another aspect of the present invention a brushless permanent magnet DC motor is provided comprising the following components: A stator having first and second sides. A rotor containing permanent magnets and radially spaced with respect to the stator to cooperate with the stator. A housing containing the stator and rotor. A stator winding having a plurality of coils, and end turns. And wherein the housing is part of the stator, and the stator/housing has integral, radially extending, circumferentially spaced cooling fins, so that there is no thermal gap, or thermal contact, resistance between distinct housing and stator components. The details of the housing, rotor, and the like preferably are as described above.
The invention also relates to a stator assembly for a brushless permanent magnet DC motor. The stator assembly comprises the following components: A stator having first and second sides. A stator winding having a plurality of coils, and end turns. And the end turns of the winding distributed on both the first and second sides of the stator, and the coils of the winding being unconnected to each other by distinct interconnection wires, interconnections between the coils comprising part of the coils. The details of the windings are preferably as described above.
The invention also relates to a method of winding a stator for a brushless permanent magnet DC motor, the stator having first and second ends, a given thickness, and slots. The method comprises the steps of: (a) Preparing a wire bundle from multiple small size copper wires by laying out a wire having a length L for one branch of a phase, and forming a circle from the wire, the circle having a diameter of L/(N.pi.), where N is the number of turns per half slot. (b) Placing the wire in the form of the circle in one of the slots of the stator with so that some of the wire of the circle is on each side of the stator with a given winding span. (c) Using the same pattern, placing all the wire of the bundle, formed into circles, into slots so that the end turns of the windings are on both said first and second sides of the stator. And (d) repeating steps (a)-(c) for the winding coils of one or more other phases so that there are no distinct interconnection wires between coils, interconnections between the coils comprising part of the coils.
In one preferred form of the invention the slots include slots 1 through 12 (a different number can be provided), and each phase coil includes first and second leads. In this situation steps (a) through (c) are practiced so that the first lead of the first phase coil is in slot 1, and the second lead of the first phase coil is in slot 10. For the second and third phase coils, the first lead of the second coil is in slot 2 and the second lead of the second coil in slot 11, and the first lead of the third coil is in slot 3, and the second lead of the third coil in slot 12. Typically steps (a) through (d) are practiced using varnished copper wire of the size between 26-32 AWG.
It is a primary object of the present invention to provide a novel winding configuration, and a motor and stator assembly constructed thereby, and a motor structure with enhanced cooling and lower cost than conventional motors for traction drive systems. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.